Abstract
Abstract. We studied gouge from an upper-crustal, low-offset reverse fault in slightly overconsolidated claystone in the Mont Terri rock laboratory (Switzerland). The laboratory is designed to evaluate the suitability of the Opalinus Clay formation (OPA) to host a repository for radioactive waste. The gouge occurs in thin bands and lenses in the fault zone; it is darker in color and less fissile than the surrounding rock. It shows a matrix-based, P-foliated microfabric bordered and truncated by micrometer-thin shear zones consisting of aligned clay grains, as shown with broad-ion-beam scanning electron microscopy (BIB-SEM) and optical microscopy. Selected area electron diffraction based on transmission electron microscopy (TEM) shows evidence for randomly oriented nanometer-sized clay particles in the gouge matrix, surrounding larger elongated phyllosilicates with a strict P foliation. For the first time for the OPA, we report the occurrence of amorphous SiO2 grains within the gouge. Gouge has lower SEM-visible porosity and almost no calcite grains compared to the undeformed OPA. We present two hypotheses to explain the origin of gouge in the Main Fault: (i) authigenic generation consisting of fluid-mediated removal of calcite from the deforming OPA during shearing and (ii) clay smear consisting of mechanical smearing of calcite-poor (yet to be identified) source layers into the fault zone. Based on our data we prefer the first or a combination of both, but more work is needed to resolve this. Microstructures indicate a range of deformation mechanisms including solution–precipitation processes and a gouge that is weaker than the OPA because of the lower fraction of hard grains. For gouge, we infer a more rate-dependent frictional rheology than suggested from laboratory experiments on the undeformed OPA.
Highlights
Gouge is a fine-grained fault rock common in near-surface faults (Sibson, 1977; Vrolijk and van der Pluijm, 1999)
We present two hypotheses to explain the origin of gouge in the Main Fault: (i) “authigenic generation” consisting of fluid-mediated removal of calcite from the deforming Opalinus Clay formation (OPA) during shearing and (ii) “clay smear” consisting of mechanical smearing of calcite-poor source layers into the fault zone
Arguments against a smear-only hypothesis are that (i) such a source layer in the faulted stratigraphy (< 80 m) of the Main Fault has not yet been identified, (ii) there is a parity in mineral composition and (iii) there is a parity in density grain size distributions (GSDs) in gouge and the undeformed OPA for grains > 10 000 nm2 (Fig. 12), and (iv) geochemical studies indicate fluid-driven mineral trans- and neoformations contemporaneous to the inferred tectonic activity (Clauer et al, 2017)
Summary
Gouge is a fine-grained fault rock common in near-surface faults (Sibson, 1977; Vrolijk and van der Pluijm, 1999). It influences the mechanical and hydraulic properties of a fault, can act as a fluid barrier or conduit and typically forms a zone of mechanical weakness, which localizes further faulting. As a follow-up to our publications on micrometer-thin shear zones and on scaly clay from the Main Fault in the Mont Terri underground research laboratory (MT URL) (Laurich et al, 2014, 2017; Laurich, 2015), in this paper we address the mineralogy and microstructure of gouge from the Main Fault to infer physical rock properties, underlying deformation mechanisms and the evolution of the gouge. We cannot cite all of these, we include a categorized literature overview in the Supplement (without a claim for completeness)
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